Ultrasonic energy is transmitted by vibrations at a frequency above the range of human hearing. Consequently ultrasonic energy cannot be humanly perceived. Ultrasonic energy is transmitted as energy waves from a variety of environmental sources, such as electrical arcs, machinery, insects and animals, and leaks in pressurized fluid systems, as well as from ultrasonic wave generators.
Different sources may produce ultrasonic waves at different frequencies. In general the amplitude of the ultrasonic waveform diminishes with the distance from its source, typically in a logarithmic or exponential relationship. This relationship makes it difficult to precisely identify ultrasonic sources at significant distances. In addition, background noise may contribute energy to the ambient environment to make detection of a specific source of ultrasonic energy more difficult or impossible, particularly at significant distances. For example, a small source of ultrasonic energy such as a small leak of gas or liquid, may be almost indistinguishable from the background noise.
Ultrasonic energy detectors are used to search for and locate sources of ultrasonic energy, since such sources cannot be humanly perceived. Sometimes, an ultrasonic wave generator is placed inside an enclosure and the detector is moved over the outside of the enclosure to locate leaks. In other cases, the escape of the pressurized fluid from a container can itself generate ultrasonic energy which may be detected without the use of a separate ultrasonic generator.
A particular ultrasonic energy source may be located by detecting the frequency associated with the particular source and distinguishing it from sources at other frequencies, or by distinguishing changes in the intensity of the detected energy relative to the physical location of the detector relative to the leak or source. The difficulties of rapid and effective detection are particularly acute in an environment where other competing ultrasonic energy sources are present. For example, a pump used to create a vacuum in a pressure vessel may emit ultrasonic energy at a particular ultrasonic frequency, and the ultrasonic energy emitted from the pump may mask the ultrasonic energy emitted from a pressure leak in the vessel. These difficulties can be exacerbated, if other ambient ultrasonic noises are present which must be distinguished while searching for the energy source.
There are a variety of ultrasonic energy detectors available to detect ultrasonic energy, but in many circumstances, these prior art detectors fail to provide enough information to accurately and quickly locate the source of the ultrasonic energy. The typical previous detector derives information regarding only one of either the amplitude or frequency of the ultrasonic source. Often, detecting only the amplitude or only the frequency is insufficient to efficiently locate and detect the ultrasonic source such as a leak.
Amplitude information may be influenced by background noise, which may obscure the true source of the ultrasonic energy, particularly when the magnitude of the ultrasonic energy is small compared to the magnitude of the background noise. The background noise may be such a significant component that it is impossible to distinguish between the background noise and the ultrasonic energy source. Visual or audible displays of the detected energy from such detectors typically contain so much background noise that they fail to accurately represent the amount of ultrasonic energy emitted from the source.
Frequency information is also available from some prior art ultrasonic detectors. The prevalent technique used to derive frequency information in ultrasonic frequency detectors is heterodyning. Heterodyning is a technique of mixing the detected ultrasonic signal with a second fixed frequency signal to obtain a "beat" signal having frequency equal to the difference between the two mixed signals. By careful selection of the fixed frequency signal relative to the expected range of the frequency of the detected ultrasonic signal, the resulting beat signal is in the audible range, thereby facilitating its human recognition.
One drawback of heterodyning is that the fixed frequency and the detected signals must occupy a relatively limited frequency relationship, or otherwise the beat signal will be outside of the audible range and therefore imperceptible. For example, the ultrasonic frequency range from 20 kHz to 200 kHz is 180 kHz in width. If signals in this range are heterodyned with a 200 kHz signal, difference frequencies from 0 to 180 kHz are produced, but only those frequencies up to 20 kHz are perceptible because they are within the audio range. Therefore, if the frequency of the source of emitted ultrasonic energy is not close to the fixed frequency, no perceptible frequency information will be produced.
Another drawback to heterodyning is that the resultant beat signal is representative of the frequency characteristics of the detected ultrasonic signal only over a relatively narrow frequency range of not greater than 20 kHz. For example if the detected ultrasonic signal continually varies in frequency over a range of 50 kHz due to variable effects at the leak in the pressure vessel or from other sources, the maximum variation in frequency which is perceivable is 20 kHz of the 50 kHz range. Furthermore with heterodyning, there is no proportional relationship between the frequency of the detected ultrasonic signal and the beat signal over the full ultrasonic range of possible detected signals. Another disadvantage of heterodyning is that the circuitry required is relatively complex and expensive.
Although frequency division has been used in ultrasonic detectors, detectors utilizing frequency division still do not provide sufficient information which simulates the normal frequency and amplitude information typical of audible sources. Consequently, ultrasonic leaks and other ultrasonic energy sources are sometimes difficult to detect efficiently and effectively with prior art detectors. It is with respect to this and other background information that the present invention has resulted.